In addition to optical recording media such as reproduction-only DVDs (digital versatile discs), recordable DVDs (DVD+RW, DVD+R, DVD-RW, DVD-RAM etc.) have been put to practical use.
The DVD+R, DVD+RW and the like are positioned as an extension of technologies of conventional recordable CD-R and recordable CD-RW (recordable compact disc). However, DVDs+R and DVDs+RW are designed so that the recording density (track pitch, signal mark length) and its substrate thickness and the like can meet DVD conditions from CD conditions to secure reproduction compatibility with reproduction-only DVDs. For example, a DVD+R is produced similarly to a CD-R, by forming a recording layer on a substrate and laminating an information recording substrate with a reflective layer formed on the recording layer with another substrate having the same shape as the information recording substrate. In the recording layer, an organic dye-based material is used.
One of characteristics of CD-R is that it has a high-reflectance (65%) which satisfies the CD standard. To obtain a high-reflectance with the above-mentioned configuration, the recording layer needs to satisfy a specific complex refractive index with a wavelength of recording and reproduction light. The reason for using an organic dye-based material in a recording layer of CD-R is that light absorption properties of an organic dye-based material are more suitable than those of an inorganic material. Light absorption properties are also needed in DVDs.
Further, in the area of reproduction-only DVDs, a reproduction-only DVD having two recording layers is proposed to increase its recording capacity. For example, FIG. 1 is a cross-sectional view showing a DVD structure having two recording layers. A first substrate 101 and a second substrate 102 are laminated to each other with sandwiching an intermediate layer 105 formed of an ultraviolet curable resin therebetween. On the inner surface of the first substrate 101, a first recording layer 103 is formed, on the inner surface of the second substrate 102, a second recording layer 104 is formed. The first recording layer 103 is formed as a semi-transparent film and is formed using a dielectric film and the like. The second recording layer is formed as a reflective film and is formed of a metal film. On the surface of the first recording layer 103, convexoconcave recording marks are formed, and recording signals are read by an effect of reflecting and interfering a reproduction laser beam. Since in the DVD illustrated in FIG. 1, signals are read from the two recording layers, a storage capacity can be obtained at a maximum of about 8.5 GB.
Here, the thickness of the first substrate 101 and the thickness of the second substrate 102 are respectively about 0.6 mm, and the thickness of the intermediate layer 105 is about 50 μm. The first recording layer 103 serving as a semi-transparent film is formed so as to have a reflectance of about 30%, and a laser beam irradiated to reproduce the second recording layer 104 is a reflected beam of which about 30% of the total quantity of light is reflected at the first recording layer 103 and attenuates. Then, the laser beam is reflected at the reflective film of the second recording layer 104, further attenuates at the first recording layer 103 and thereafter is emitted from the optical recording medium. Then, signals stored in the respective recording layers can be reproduced by focusing the laser beam as a reproduction beam so as to be brought to a focus on the first recording layer or the second recording layer and detecting a reflected beam. Note that in the case of a DVD, the wavelength of a recording and reproduction laser beam is about 650 nm.
Similarly to reproduction-only DVDs, in recordable DVDs, DVDs+R and DVDs-R are studied as optical recording media based on a single-sided two-layer type recording and reproducing method (for example, see Patent Literature 1).
However, with the recent development of high-speed recording systems, disc performance suitable for recording at high-linear velocity is required, however, it is difficult for conventional recording media to respond to the demand.
Specifically, a single-sided single layer recording medium of DVD is set to achieve a standard recording/reproduction linear velocity of 3.49 m/s, and a single-sided two-layer recording medium of DVD is set to achieve a standard recording/reproduction linear velocity of 3.83 m/s. In recent years, it is required to respond to achieve a recording linear velocity 12 times to 18 times higher than these standard linear velocities. Unfortunately, under such a high linear velocity condition, thermal interference of recording marks easily occurs and a signal jitter is increased.
In addition to the above, single-sided two-layer optical recording media, even though it is a single-sided two-layer optical recording medium as described in Patent Literature 1, has a problem that a sufficient recording speed and a sufficient recording sensitivity cannot be obtained as compared to conventional single-sided single-layer recording media.
The reason is that, since a single-sided two-layer recording and reproduction type optical recording medium has two recording layers, when signals are recorded with irradiating a rewriting laser beam by focusing the laser beam on a recording layer disposed at the outermost position from an optical pickup, the laser beam attenuates at a first recording layer, and thus it is difficult to obtain both light absorption and light reflection necessary for recording the signals on a second recording layer.
Particularly for a second recording layer, there is a problem that it is difficult to form recording marks thereon because a second recording layer has a different layer structure from those of existing CDs-R and DVDs+R. In an existing single-layer recording and reproducing medium, a substrate, a dye layer, a reflective layer and a protective layer are formed in this order as viewed from the light incident side of the medium. However, on a second recording layer side in a two-layer recording and reproducing medium, a laminated layer, (an organic protective layer), a recording layer, a reflective layer and a substrate are formed in this order as viewed from the light incident side (as viewed from a laser beam that has been optically transmitted to the first recording layer). Thus, the surrounding environment of forming recording marks on a second recording layer (adjacent layer) differs between a two-layer recording and reproducing medium and a CD-R or the like. For this reason, it is difficult to control of forming recording marks when a recording linear velocity is high, and recording and reproducing properties such as a modulation degree and a jitter that can be used in DVD systems are hardly obtained.
In the meanwhile, as dye materials for DVD recording layer, cyanine dyes, azo dyes, squarylium dyes and the like have been put into practical use. Of these materials, as a dye material suitable for high-speed recording performance, cyanine dyes are proposed (for example, see Patent Literature 2 and Patent Literature 3). However, cyanine dyes are not capable of securing a practical durability. Specifically, it is difficult for a cyanine dye to retain data in a disc durability test under high-temperature and high-humidity environment such as 80° C.-85% RH. Accordingly, the current situation is that it is desired to immediately develop a single-sided single-layer optical recording medium that is excellent in durability and enables obtaining excellent signal properties even at a recording speed of 12 times to 18 times higher than the standard recording speed and to develop a single-sided two-layer recording and reproduction type optical recording medium.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2006-44241    Patent Literature 2 Japanese Patent (JP-B) No. 3698708    Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2005-205874